CN106066287A - The decision method of titanium alloy fatigue crack growth rate curve Paris district turning point - Google Patents

The decision method of titanium alloy fatigue crack growth rate curve Paris district turning point Download PDF

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CN106066287A
CN106066287A CN201610347444.1A CN201610347444A CN106066287A CN 106066287 A CN106066287 A CN 106066287A CN 201610347444 A CN201610347444 A CN 201610347444A CN 106066287 A CN106066287 A CN 106066287A
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fracture
fatigue crack
growth rate
crack growth
curve
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祝力伟
朱知寿
王新南
商国强
费跃
李静
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BEIJING INSTITUTE OF AERONAUTICAL MATERIALS CHINA AVIATION INDUSTRY GROUP Corp
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0001—Type of application of the stress
    • G01N2203/0005—Repeated or cyclic
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058—Kind of property studied
    • G01N2203/006—Crack, flaws, fracture or rupture
    • G01N2203/0062—Crack or flaws
    • G01N2203/0066—Propagation of crack

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Abstract

The invention belongs to fatigue of materials cracks can spread behavioral analysis technology field, it is provided that the decision method of a kind of titanium alloy fatigue crack growth rate curve Paris district turning point.Cracks can spread behavior that the method is shown according to fracture microscopic feature and the change of fracture mode, determine that Fatigue Crack Propagation Mechanism occurs crack extending length when changing, by solving the stress intensive factor range corresponding to this crack extending length and fatigue crack growth rate, then judge the turning point position in fatigue crack growth rate curve Paris district.Scientific basis of the present invention is abundant, and degree of accuracy is high, can meet design user and carry out material property evaluation and damage tolerance design, the beneficially demand of modernization project application.

Description

The decision method of titanium alloy fatigue crack growth rate curve Paris district turning point
Technical field
The invention belongs to fatigue crack propagation analysis technical field, relate to a kind of titanium alloy fatigue crack growth rate The decision method of curve Paris district turning point.
Background technology
Common fatigue crack growth rate curve generally can be divided into three in log-log coordinate da/dN-Δ K relation curve In the individual stage, Δ K during the lowest Δ K scope is bordering on threshold value (Δ Kth) stage (Ith district), middle Δ K scope time stable extension rank When section (IIth district, i.e. Paris district) and high Δ K scope, Δ K quickly tends to fracture toughness (Δ KIC) stage (IIIth district).Numerous studies are tied Fruit shows, fatigue crack growth rate curve has log-linear relation in Paris district, i.e. log-log coordinate da/dN-Δ K is bent Line is a straight line in Paris district, utilizes this relation to carry out fatigue life prediction and durability analysis, is material structure fatigue study Emphasis.The fatigue crack propagation of damage tolerance Type Titanium Alloy is studied discovery, its log-log coordinate da/dN-Δ K curve There is obvious turning point phenomenon in Paris district, turning point forward and backward cracks can spread behavior and fracture mode change so that Fatigue crack growth rate curve shows non-linear relation in Paris district, if be fitted with Paris formula, it will make There is relatively large deviation in fitting result and test data, and the biggest the closer to deviation near threshold value.Research shows, for above-mentioned Test data does not meets the situation of linear rule, carries out piecewise fitting with turning point for separation, it is possible to obtain ideal Fitting effect, therefore, accurately judges curve break position, Paris district, obtains corresponding stress intensity factor range Δ K, For Calculation of Fatigue Life and damage tolerance design, engineering has great importance.
At present, the decision method of the fatigue crack fatigue crack growth rate curve turning point of existing document report mainly includes mesh Survey method and mathematical method.Ocular estimate is when determining the position of curve break, and error is relatively big, and result exists bigger with practical situation Deviation；The mathematical approach includes the decision methods such as intercept method, iterative method and curvature method, can be used for judging have simple shape curve Turning point, but mathematical method theoretical foundation is not enough, and required result is actual not to be inconsistent.Additionally, when test data is complicated, data When dispersibility is bigger, test data curve setback are relatively big, often have multiple turning point, and now, ocular estimate, mathematical method are in standard Really judging all to encounter difficulty in the problem of turning point, can only judge according to engineering experience, result often lacks verity And effectiveness, accordingly, it would be desirable to develop a kind of analysis method that can accurately judge fatigue crack growth rate curve turning point.
Summary of the invention
The technical problem to be solved is for above-mentioned the deficiencies in the prior art, it is provided that one is more accurate The method determining titanium alloy fatigue crack growth rate curve turning point.For solving above-mentioned technical problem, the skill that the present invention uses Art scheme is:
Step one, carry out titanium alloy fatigue crack growth rate test, record i & lt load cycle number NiAnd NiInstitute Corresponding crack extending length ai, obtain i test data point (N altogetheri,ai)；
Step 2, the test data (N obtained according to step onei,ai), draw a-N curve；
Step 3, the specimen geometry tested according to fatigue crack growth rate, calculate aiCorresponding stress intensity because of Subrange Δ Ki, differentiate according to the a-N curve in step 2, obtain crack growth rate (da/dN)i, then, draw fatigue Fatigue crack growth rate curve, i.e. log-log coordinate da/dN-Δ K curve；
Step 4, utilize any fracture length L of electron microscope observationiThe fracture micro-appearance feature at place, analyzes arbitrarily Fracture length LiCorresponding cracks can spread behavior and fracture mode；
Step 5, judge any fracture length LiWhether corresponding cracks can spread behavior and fracture mode are organized by low stress Sensitizing range changes to steady propagation district:
If fracture length LiCorresponding fatigue crack extends in pure shear mode, and fracture apperance is coarse, crack propagation path Complications, fracture mode shows as along colony interface or intercrystalline fracture, and cracks can spread behavior and fracture mode now show as Low-stress undertissue sensitizing range feature, cracks can spread behavior and fracture mode change, and perform step 4, to fracture Length is more than LiDirection observe,
If fracture length LiCorresponding fatigue crack extends with polyphyly slip mode, and fracture apperance is smooth, cracks can spread road Footpath is straight, and fracture mode shows as passing through colony or transgranular fracture feature, cracks can spread behavior now and fracture mode performance For steady propagation district feature, cracks can spread behavior and fracture mode change, perform step 4, to fracture length less than Li Direction observe,
If fracture length LiCorresponding fatigue crack changes to polyphyly slip mode in pure shear mode, cracks can spread behavior With fracture mode by along colony interface or intercrystalline fracture to passing through colony or transgranular fracture changes, now fracture length LiCorresponding Fracture length LtIt is cracks can spread behavior and the position of fracture mode generation transformation, performs step 6；
Step 6, the specimen geometry tested according to fatigue crack growth rate, calculate fracture length LtCorresponding should Force intensity factor range Δ Kt；
Step 7, fitting of a polynomial a-N curve derivation, calculate fracture length LtCorresponding fatigue crack growth rate (da/ dN)t；
Step 8, point (Δ Kt,(da/dN)t) it is the turning point in fatigue crack growth rate curve Paris district.
Described fracture length LtCorresponding stress intensity factor range Δ Kt, can be by stress intensity factor range expression formula Determine.
Described fracture length LtCorresponding fatigue crack growth rate (da/dN)t, can be by incremental polynomials local Matching a-N curve derivation determines
During described incremental polynomials local fit a-N curve, test data point (Ni,ai) crack extending length aiWith Fracture length LiThe absolute value of difference minimum, i.e. | ai-1-Lt| ＜ | ai-Lt| ＜ | ai+1-Lt| (i ＞ 3), and ai-3＜ Lt＜ ai+3
The present invention compared with prior art has the advantage that
(1) in the nearly Threshold Region of low stress of crack Propagation, crackle extends in pure shear mode, and fracture is the most coarse, Crack propagation path is tortuous, and fracture mode shows as along colony interface or crystal boundary face fracture characteristic, and now cracks can spread is in low Speed extension phase, crack growth rate is slower；In heavily stressed steady propagation district, fatigue crack extends with polyphyly slip mode, disconnected Mouth is more mild, and crack propagation path is straight, and fracture mode shows as passing through colony or transgranular fracture feature, now cracks can spread Being in the steady propagation stage, crack growth rate is very fast.Owing to fatigue crack propagation and fracture mode directly determine tired Labor crack growth rate so that the fatigue crack growth rate under different damage and failure mechanisms varies in size, and reflection is split to fatigue On stricture of vagina spreading rate curve, i.e. showing as slope of a curve existence and be clearly distinguished from, curve certainly exists a turning point.The present invention By analyzing cracks can spread behavior and the change of fracture mode that fracture micro-appearance feature is shown, judge fatigue crack Spreading rate curve is in the turning point position in Paris district, it is possible to the actual physics process of true reflection crack Propagation and power Scholarship and moral conduct is, scientific basis is abundant, it is determined that result has higher accuracy and effectiveness than traditional ocular estimate, mathematical method；
(2) present invention is with the turning point position that determines as separation, and fatigue crack growth rate curve is carried out segmentation plan Close, it is possible to obtain ideal fitting effect；
(3) technical solution of the present invention is complete, and scientific basis is abundant, and accuracy is high, calculates for structure fatigue life and damage Tolerance design provides a kind of effective analytical tool, and beneficially modernization project application needs.
Accompanying drawing explanation
The flow process of a kind of method determining titanium alloy fatigue crack growth rate curve turning point that Fig. 1 provides for the present invention Figure；
Fig. 2 is the fatigue crack growth rate test a-N curve of titanium alloy involved in the present invention；
Fig. 3 be titanium alloy involved in the present invention fatigue crack growth rate log-log coordinate da/dN-Δ K curve and Paris expression formula；
Fig. 4 is the fracture microscopic feature of titanium alloy fatigue crack growth rate involved in the present invention test；
Fig. 5 is turning point and the Paris thereof in titanium alloy fatigue crack growth rate curve Paris district determined by the present invention Parting expression.
Detailed description of the invention
The stream of the decision method of a kind of titanium alloy fatigue crack growth rate curve Paris district turning point as shown in Figure 1 Cheng Tu, the present invention is the cracks can spread behavior and fracture mode shown by electron-microscopic analysis fracture microscopic feature, Determine that Fatigue Crack Propagation Mechanism occurs fracture length L corresponding when changingt, calculate fracture length LtCorresponding stress intensity because of Subrange Δ KtWith fatigue crack growth rate (da/dN)t, thus judge titanium alloy fatigue crack growth rate curve Paris district Turning point, detailed description of the invention can follow the steps below:
Step one, preparation start.The present invention be applicable to the nearly α type such as TC4-DT, TC21, TA15ELI, Ti-6Al-4V ELI, Alpha-beta Type Titanium Alloy, such alloy obtains lamellar structure, basket organizational structure, its double-log after certain forging and heat treatment Turnover phenomenon is there is in coordinate da/dN-Δ K curve in Paris district；
The standards such as step 2, foundation GB/T 6398-2000, HB 5216-1998, О С Т 1 90268-78 are carried out titanium and are closed Gold fatigue crack growth rate test, records several load cycle numbers and the crack length of correspondence thereof in process of the test, To i-th load cycle number NiAnd NiCorresponding crack extending length ai, it is thus achieved that i test data point (Ni,ai)。
Step 3, the test data (N that step 2 is obtainedi,ai) be plotted on coordinate axes, obtain crack extending length ai With load cycle times NiRelation curve, i.e. a-N curve；
Step 4, the specimen geometry tested according to fatigue crack growth rate, calculate crack extending length aiCorresponding Stress intensity factor range Δ Ki, wherein, the sample of fatigue crack growth rate test includes standard C (T) sample, standard M (T) sample and standard SE (B) sample, the stress intensity factor range Δ K of its correspondenceiMathematic(al) representation according to GB/T 6398- 2000, the regulation of the standard such as HB 5216-1998, О С Т 1 90268-78；By the a-N curve derivation in fit procedure three Method determine crack extending length aiCorresponding fatigue crack growth rate (da/dN)i, according to GB/T 6398-2000, HB Secant method or incremental polynomials method that the standards such as 5216-1998, О С Т 1 90268-78 specify determine.By calculated Stress intensity factor range Δ KiWith fatigue crack growth rate (da/dN)iIt is plotted on log-log coordinate axle, i.e. obtains biconjugate Number coordinate da/dN-Δ K curve；
Step 5, utilize any fracture length L of electron microscope observationiThe fracture micro-appearance feature at place, analyzes arbitrarily Fracture length LiCorresponding cracks can spread behavior and fracture mode.
Result of study shows, in Paris district, existence turns the log-log coordinate da/dN-Δ K curve of titanium alloy of the present invention Break phenomenon, the curve in Paris district is divided into two parts by turning point, and turning point is forward and backward shows different cracks can spread behaviors And fracture mode.Low stress area before turning point, fatigue crack growth rate curve fluctuation is relatively big, and cracks can spread is to micro- Tissue is more sensitive, and fracture is the most coarse, and crack propagation path is tortuous, and fatigue crack extends in pure shear mode, fracture mode Show as, along colony interface or crystal boundary face fracture characteristic, belonging to the district of low stress tissue sensitivity of cracks can spread；After turning point Fatigue crack growth rate curve is substantially aligned, and fracture is more mild, and crack propagation path is straight, and fatigue crack is with polyphyly Slip mode extends, and fracture mode shows as passing through colony or transgranular fracture feature, belongs to the steady propagation of crack Propagation Stage.Above-mentioned physical process shows on fatigue crack growth rate curve, i.e. at log-log coordinate da/dN-Δ K curve , there is a turning point in Paris district, the curve in Paris district is divided into two parts by this, two parts slope of a curve before and after this shop Existence is clearly distinguished from.
The analysis of causes formed according to above-mentioned fatigue crack growth rate curve turning point, for titanium alloy of the present invention The result of fatigue crack growth rate test, utilizes any fracture length L of electron microscope observationiCorresponding cracks can spread behavior With fracture microscopic feature, analyze fracture length LiCorresponding cracks can spread behavior and fracture mode.
Step 6, judge any fracture length LiWhether corresponding cracks can spread behavior and fracture mode are organized by low stress Sensitizing range changes to steady propagation district:
If fracture length LiCorresponding fatigue crack extends in pure shear mode, and fracture apperance is coarse, crack propagation path Complications, fracture mode shows as along colony interface or intercrystalline fracture, and cracks can spread behavior and fracture mode now show as Low-stress undertissue sensitizing range feature, cracks can spread behavior and fracture mode change, and perform step 5, to fracture Length is more than LiDirection observe；
If fracture length LiCorresponding fatigue crack extends with polyphyly slip mode, and fracture apperance is smooth, cracks can spread road Footpath is straight, and fracture mode shows as passing through colony or transgranular fracture feature, cracks can spread behavior now and fracture mode performance For steady propagation district feature, cracks can spread behavior and fracture mode change, perform step 5, to fracture length less than Li Direction observe；
If fracture length LiCorresponding fatigue crack changes to polyphyly slip mode in pure shear mode, cracks can spread behavior With fracture mode by along colony interface or intercrystalline fracture to passing through colony or transgranular fracture changes, now fracture length LiCorresponding Fracture length LtIt is cracks can spread behavior and the position of fracture mode generation transformation, performs step 7；
Step 7, the specimen geometry tested according to fatigue crack growth rate, calculate fracture length LtCorresponding should Force intensity factor range Δ Kt.Fatigue crack growth rate test sample include standard C (T) sample, standard M (T) sample and Standard SE (B) sample, the stress intensity factor range Δ K of its correspondenceiAccording to GB/T 6398-2000, HB 5216-1998, О The regulation of the standards such as С Т 1 90268-78 calculates；
Step 8, fitting of a polynomial a-N curve derivation, calculate fracture length LtCorresponding fatigue crack growth rate (da/ dN)t.From total Test data point (Ni,aiSelect (2n+1) individual consecutive numbers strong point in), use quadratic polynomial to carry out local and intend Close, the quadratic polynomial obtained is carried out derivation, is calculated fracture length LtCorresponding fatigue crack growth rate (da/dN )t.Consecutive numbers strong point (N individual for (2n+1)i,ai) selection principle be: total Test data point (Ni,aiCracks can spread in) Length aiWith fracture length L determined in step 6iThe absolute value of difference minimum, i.e. | ai-1-Lt| ＜ | ai-Lt| ＜ | ai+1-Lt|, A by satisfied requirementiCorresponding data point (Ni,ai) it is defined as i-th point, before and after taking it, adjacent nearest each n point, adds Upper i point itself, i.e. obtains (2n+1) individual consecutive numbers strong point, n value desirable 2, and 3,4, typically take 3.
Step 9, on log-log coordinate system da/dN-Δ K curve, determine point (Δ Kt,(da/dN)t) position, this point is i.e. For the fatigue crack growth rate curve turning point in Paris district.
Step 10, end.
Embodiment
Step one, preparation start.The present embodiment with in strong high-ductility high-damage tolerance type TC4-DT titanium alloy as object, often Rule two-phase section forging and quasi-β heat treatment after, it is thus achieved that microscopic structure be typical lamellar structure, the composition of material and mechanical property Such as Tables 1 and 2.
Table 1 material composition
Table 2 mechanical property
Step 2, carry out fatigue crack growth rate test.According to " Fatigue Crack Growth Rate of Metallic Materials test side Method " (GB/T 6398-2000) carry out fatigue crack growth rate test, and experimental enviroment is room temperature, air, test frequency f= 10Hz, maximum load Pmax=6.4KN, stress ratio R=0.1.I-th load cycle number N is recorded in process of the testiAnd NiRight The crack extending length a answeredi, it is thus achieved that total Test data point (Ni,ai)；
Step 3, the test data (N that step 2 is obtainedi,ai) be plotted on coordinate axes, obtain crack extending length ai With load cycle times NiRelation curve, i.e. a-N curve, as shown in Figure 2；
Step 4, the specimen geometry tested according to fatigue crack growth rate, calculate aiCorresponding stress intensity because of Subrange Δ Ki, differentiate according to the a-N curve in step 2, obtain crack growth rate (da/dN)i, then, draw fatigue Fatigue crack growth rate curve, i.e. log-log coordinate da/dN-Δ K curve.
The secant method provided according to GB/T 6398-2000, the straight line calculating adjacent two data point on a-N curve is oblique Rate, its expression formula is
( d a d N ) a → = ( a i + 1 - a i ) / ( N i + 1 - N i ) - - - ( 1 )
In formula, aiFor the crack length after i & lt load cycle, ai+1For the crack length after i+1 time load cycle,For increment (ai+1-ai) average crack growth rates, average crack growth lengthFor calculating Stress intensity factor range Δ Ki。
The sample that the present embodiment selects is standard C (T) sample, sample initial incision length a0=10mm, several according to sample What shape, the stress intensity factor range Δ K that standard GB/T 6398-2000 specifiesiMathematic(al) representation be
ΔK i = Δ P B W ( 2 + α ) ( 1 - α ) - 3 / 2 ( 0.886 + 4.64 α - 13.32 α 2 + 14.72 α 3 - 5.6 α 4 ) - - - ( 2 )
In formula, α=a/W, a=a0+ai, and a/W >=0.2, a0For sample initial incision length, aiFor average crack growth LengthB is sample thickness, and W is specimen width, and Δ P is load range.
According to formula (1) and formula (2), calculating stress strength factor range delta KiWith fatigue crack growth rate (da/ dN)i, log-log coordinate fatigue crack growth rate curve can be drawn, i.e. obtain log-log coordinate da/dN-Δ K curve, such as Fig. 3 Shown in.It can be seen that there is obvious turning point in Paris district in the log-log coordinate da/dN-Δ K curve of TC4-DT titanium alloy Phenomenon, the slope existence of the forward and backward fatigue crack growth rate curve of turning point is clearly distinguished from, and curve shows non-in Paris district Linear rule.
If directly utilizing Paris model to be fitted, its mathematic(al) representation is
d a d N = C · ΔK m = 8.5212 × 10 - 11 ( Δ K ) 4.4393
Directly utilize fit equation that Paris model the obtains fitting effect to test data as it is shown on figure 3, can see Going out, the Paris fit equation of the log-log coordinate da/dN-Δ K curve of TC4-DT titanium alloy is straight line, although can retouch State the cracks can spread behavior in steady propagation district, but the closer to threshold value region, fit equation is the biggest with test data deviation.Cause This, directly use Paris equation to be fitted test data, it is impossible to describe the fatigue crack propagation of nearly Threshold Region, it was predicted that It is worth higher than test value so that life estimate result is the safest.
When above-mentioned test data does not meets linear rule, carry out piecewise fitting with turning point for separation, can To obtain ideal fitting effect, therefore, the fatigue crack growth rate curve turning point position in Paris district is accurately judged Put, there is important engineering significance.Test data involved by the present embodiment is more complicated, uses ocular estimate, mathematical method In the problem accurately judging turning point position, encounter difficulty etc. traditional method, can only judge according to engineering experience, past Toward causing judged result and actual physics process to there is deviation, lack verity and effectiveness, accordingly, it would be desirable to development one can True reflection fatigue crack propagation and the analysis method of actual physics process, make the turning point position of acquisition have verity And effectiveness；
Step 5, utilize any fracture length L of electron microscope observationiThe fracture micro-appearance feature at place, analyzes arbitrarily Fracture length LiCorresponding cracks can spread behavior and fracture mode.
Result of study shows, in Paris district, existence turns the log-log coordinate da/dN-Δ K curve of titanium alloy of the present invention Break phenomenon, the curve in Paris district is divided into two parts by turning point, and turning point is forward and backward shows different cracks can spread behaviors And fracture mode.Low stress area before turning point, fatigue crack growth rate curve fluctuation is relatively big, and cracks can spread is to micro- Tissue is more sensitive, and fracture is the most coarse, and crack propagation path is tortuous, and fatigue crack extends in pure shear mode, fracture mode Show as, along colony interface or crystal boundary face fracture characteristic, belonging to the district of low stress tissue sensitivity of cracks can spread；After turning point Fatigue crack growth rate curve is substantially aligned, and fracture is more mild, and crack propagation path is straight, and fatigue crack is with polyphyly Slip mode extends, and fracture mode shows as passing through colony or transgranular fracture feature, belongs to the steady propagation of crack Propagation Stage.Above-mentioned physical process shows on fatigue crack growth rate curve, i.e. at log-log coordinate da/dN-Δ K curve , there is a turning point in Paris district, the curve in Paris district is divided into two parts by this, two parts slope of a curve before and after this shop Existence is clearly distinguished from.
The analysis of causes formed according to above-mentioned fatigue crack growth rate curve turning point, for titanium alloy of the present invention The result of fatigue crack growth rate test, utilizes any fracture length L of electron microscope observationtCorresponding cracks can spread behavior With fracture microscopic feature, result is as shown in Figure 4；
Step 6, analysis result according to step 5, it is judged that arbitrarily fracture length LiCorresponding cracks can spread behavior is with disconnected Split whether mode is changed to steady propagation district by district of low stress tissue sensitivity:
If fracture length LiCorresponding fatigue crack extends in pure shear mode, and fracture apperance is coarse, crack propagation path Complications, fracture mode shows as along colony interface or intercrystalline fracture, and cracks can spread behavior and fracture mode now show as Low-stress undertissue sensitizing range feature, cracks can spread behavior and fracture mode change, and perform step 5, to fracture Length is more than LiDirection observe；
If fracture length LiCorresponding fatigue crack extends with polyphyly slip mode, and fracture apperance is smooth, cracks can spread road Footpath is straight, and fracture mode shows as passing through colony or transgranular fracture feature, cracks can spread behavior now and fracture mode performance For steady propagation district feature, cracks can spread behavior and fracture mode change, perform step 5, to fracture length less than Li Direction observe；
If fracture length LiCorresponding fatigue crack changes to polyphyly slip mode in pure shear mode, cracks can spread behavior With fracture mode by along colony interface or intercrystalline fracture to passing through colony or transgranular fracture changes, now fracture length LiCorresponding Fracture length LtIt is cracks can spread behavior and the position of fracture mode generation transformation, performs step 7.Described in the present embodiment Fracture length L corresponding when changing is there is in titanium alloy cracks can spread behavior with fracture modet=25.29mm, performs step 7；
Step 7, the specimen geometry tested according to fatigue crack growth rate, calculate fracture length LtCorresponding should Force intensity factor range Δ Kt.Fatigue crack growth rate of the present invention test employing standard compact tension specimen C (T) sample, examination Sample thickness B=12.5mm, specimen width W=50mm, incision length a0=10mm, maximum load Pmax=6.4KN, stress ratio R= 0.1, load range Δ P=Pmax(1-R)=5.76KN, is calculated fracture length L by formula (2)tCorresponding stress intensity factor Scope
Step 7, fitting of a polynomial a-N curve derivation, calculate fracture length LtCorresponding fatigue crack growth rate (da/ dN)t.From total Test data point (Ni,aiSelect (2n+1) individual consecutive numbers strong point in), use quadratic polynomial to carry out local and intend Close, the quadratic polynomial obtained is carried out derivation, is calculated fracture length LtCorresponding fatigue crack growth rate (da/dN )t.The present embodiment selects n=3, for satisfactory i-th test data point (Ni,ai), take before and after it the most adjacent nearest each 3 Individual, add i point totally 7 consecutive numbers strong points itself, then use quadratic polynomial local fit, the fitting formula obtained is asked Lead and determine LtCorresponding fatigue crack growth rate (da/dN)t.The decision principle at 7 consecutive numbers strong points is: total Test data Point (Ni,aiCrack extending length a in)iWith fracture length L determined in step 6iThe absolute value of difference minimum, i.e. | ai-1-Lt | ＜ | ai-Lt| ＜ | ai+1-Lt|, by a of satisfied requirementiCorresponding test data point (Ni,ai) it is defined as i-th point, take it The most adjacent nearest each 3 test data points, are 7 consecutive numbers strong points.
According to mentioned above principle, described in the present embodiment, titanium alloy fatigue crack growth rate tests the total Test number recorded Strong point (Ni,aiIn), with fracture length L that determines in step 6tThe minimum crack extending length a of the absolute value of differenceiValue is 25.31mm, therefore, is defined as i-th point by data point (1090047,25.31), adjacent nearest each 3 points before and after taking it, It is met 7 long run test data points of requirement, as shown in table 3.
The load cycle number N that 3 seven continuity points of table are correspondingiWith crack extending length ai
The present embodiment, for 7 the consecutive numbers strong points determined in table 3, uses quadratic polynomial local fit a-N curve, To fitting formula be
F (x)=A+Bx+Cx2=3291.58693-6.53 × 10-3x+3.07608×10-9x2 (3)
Formula (3) is carried out derivation, it is thus achieved that fracture length LtCorresponding crack growth rate (da/dN)t, i.e.
Step 9, on titanium alloy fatigue crack growth rate log-log coordinate da/dN-Δ K curve of the present invention really Fixed point P (20.14,9.7885 × 10-5) position, this point is the turning point of fatigue crack growth rate curve, such as Fig. 5 institute Show.
The present embodiment is corresponding with the turning point position determinedFor separation, use Paris equation Fatigue crack growth rate curve is carried out piecewise fitting, and test data is had ideal by the piecewise fitting equation obtained Fitting effect, as it is shown in figure 5, fitting precision reaches 0.9917, the mathematic(al) representation of its Paris segmentation equation is
Δ K ≤ ΔK t : d a d N = C · ΔK m = 2.66213 × 10 - 14 ( Δ K ) 7.26743 Δ K > ΔK t : d a d N = C · ΔK m = 3.98896 × 10 - 9 ( Δ K ) 3.38105
Step 10, end
The embodiment of the present invention successfully solves the decision problem of titanium alloy fatigue crack growth rate curve turning point, for Structure fatigue life calculates and damage tolerance design provides a kind of effective analytical tool.A kind of titanium alloy that the present invention proposes The decision method of fatigue crack growth rate curve Paris district turning point, technical scheme is complete, and scientific basis is abundant, accuracy Height, the beneficially demand of modernization project application.
The above, be only presently preferred embodiments of the present invention, not impose any restrictions the present invention.Every according to the present invention Any simple modification, change and the equivalence change that technical spirit is made, belongs to the protection domain of technical solution of the present invention.

Claims (4)

1. the decision method of a titanium alloy fatigue crack growth rate curve Paris district turning point, it is characterised in that the method Comprise the following steps:
Step one, carry out titanium alloy fatigue crack growth rate test, record i & lt load cycle number NiAnd NiCorresponding Crack extending length ai, obtain i test data point (N altogetheri,ai)；
Step 2, the test data (N obtained according to step onei,ai), draw a-N curve；
Step 3, the specimen geometry tested according to fatigue crack growth rate, calculate aiCorresponding stress intensity factor range ΔKi, differentiate according to the a-N curve in step 2, obtain crack growth rate (da/dN)i, then, draw fatigue crack and expand Exhibition rate curve, i.e. log-log coordinate da/dN-Δ K curve；
Step 4, utilize any fracture length L of electron microscope observationiThe fracture micro-appearance feature at place, analyzes any fracture long Degree LiCorresponding cracks can spread behavior and fracture mode；
Step 5, judge any fracture length LiWhether corresponding cracks can spread behavior and fracture mode be by low stress tissue sensitivity District changes to steady propagation district:
If fracture length LiCorresponding fatigue crack extends in pure shear mode, and fracture apperance is coarse, and crack propagation path is tortuous, Fracture mode shows as along colony interface or intercrystalline fracture, and cracks can spread behavior and fracture mode now show as low stress Under the conditions of tissue sensitivity's district's feature, cracks can spread behavior and fracture mode change, and performs step 4, big to fracture length In LiDirection observe,
If fracture length LiCorresponding fatigue crack extends with polyphyly slip mode, and fracture apperance is smooth, and crack propagation path is put down Directly, fracture mode shows as passing through colony or transgranular fracture feature, and cracks can spread behavior and fracture mode now show as surely State expansion area feature, cracks can spread behavior and fracture mode change, perform step 4, to fracture length less than LiSide To observation,
If fracture length LiCorresponding fatigue crack changes to polyphyly slip mode in pure shear mode, and cracks can spread behavior is with disconnected Split mode by along colony interface or intercrystalline fracture to passing through colony or transgranular fracture changes, now fracture length LiCorresponding is disconnected Mouth length LtIt is cracks can spread behavior and the position of fracture mode generation transformation, performs step 6；
Step 6, the specimen geometry tested according to fatigue crack growth rate, calculate fracture length LtCorresponding stress intensity Factor range Δ Kt；
Step 7, fitting of a polynomial a-N curve derivation, calculate fracture length LtCorresponding fatigue crack growth rate (da/dN)t；
Step 8, point (Δ Kt,(da/dN)t) it is the turning point in fatigue crack growth rate curve Paris district.
The judgement side of a kind of titanium alloy fatigue crack growth rate curve Paris district the most according to claim 1 turning point Method, it is characterised in that described fracture length LtCorresponding stress intensity factor range Δ Kt, can be by stress intensity factor range Expression formula determines.
The judgement side of a kind of titanium alloy fatigue crack growth rate curve Paris district the most according to claim 1 turning point Method, it is characterised in that described fracture length LtCorresponding fatigue crack growth rate (da/dN)t, can be multinomial by being incremented by Formula local fit a-N curve derivation determines.
The judgement side of a kind of titanium alloy fatigue crack growth rate curve Paris district the most according to claim 1 turning point Method, it is characterised in that during described incremental polynomials local fit a-N curve, test data point (Ni,ai) cracks can spread long Degree aiWith fracture length LiThe absolute value of difference minimum, i.e. | ai-1-Lt| ＜ | ai-Lt| ＜ | ai+1-Lt| (i ＞ 3), and ai-3＜ Lt ＜ ai+3。
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